1. Field of the Invention
The present invention relates to a motor vehicle braking system which is capable of performing a braking operation while controlling the attitude of a motor vehicle.
2. Description of Related Art
One technique for attitude control of a motor vehicle is to utilize an ABS system (anti-lock braking system) for independently controlling braking pressures on four vehicle wheels. That is, the yaw rate or lateral acceleration of the motor vehicle is controlled by the independent control of the braking pressures on the four vehicle wheels, thereby preventing attitude instability of the motor vehicle. Particularly on a low friction road surface such as an iced road surface, the attitude control through the control of the braking pressures (braking attitude control) is more effective than attitude control by a steering mechanism (steering attitude control).
The ABS system allows for an ideal braking operation while preventing the vehicle wheels from being locked to realize the minimum possible braking distance, when a braking pedal is heavily stepped down to apply a maximum braking command.
Where the behavior of the motor vehicle is to be stabilized on a so-called xcexc-split road, however, it is impossible to generate a maximum braking force. The term xe2x80x9cxcexc-split roadxe2x80x9d herein means a road having significantly different friction coefficients with respect to left and right wheels of the motor vehicle. A typical case is such that the right wheels are on a dry asphalt road surface and the left wheels are on an iced road surface. When a greater braking pressure is applied to the vehicle wheels on a high xcexc (friction coefficient) side of the xcexc-split road, a great yaw moment is generated thereby to cause the motor vehicle to easily spin. Therefore, the stabilization of the behavior of the motor vehicle should primarily be achieved by reducing the braking pressures to be applied to the vehicle wheels on the high xcexc side which can otherwise provide a greater braking force. Accordingly, there is no other way but to reduce the deceleration, whereby the braking distance and the braking time are increased.
It is an object of the present invention to provide a motor vehicle braking system which is capable of advantageously performing a braking operation on a xcexc-split road.
The motor vehicle braking system according to the present invention comprises: a first attitude control circuit (20) for stabilizing the behavior of a motor vehicle by controlling the operation of a steering mechanism (2,3) of the motor vehicle; a second attitude control circuit (60) for stabilizing the behavior of the motor vehicle by controlling a braking mechanism (53,54) for applying a braking force to vehicle wheels of the motor vehicle; and a braking control circuit (60) for causing the braking mechanism to generate a maximum braking force on condition that an attitude control is performed for the stabilization of the behavior of the motor vehicle by the first attitude control circuit when a braking command is inputted thereto. Parenthesized alphanumeric characters denote corresponding components to be described later in the embodiment of the invention, but do not intend to limit the invention to the embodiment. This definition is effectual in this section.
With the aforesaid arrangement, a braking operation is performed by the generation of the maximum braking force in response to the input of the braking command on condition that the behavior of the motor vehicle is stabilized through the control of the steering mechanism. Thus, the motor vehicle is decelerated at a higher rate with the attitude thereof being stabilized through the attitude control operation, whereby the braking distance and the braking time can be reduced.
The steering mechanism preferably has no mechanical connection to a steering operation member (1) such as a steering wheel, or is adapted to be brought out of mechanical engagement with the steering operation member as required. With this arrangement, the steering mechanism can electrically be controlled in accordance with the operation of the steering operation member, whereby a steering control can be performed as intended by a driver and the behavior of the motor vehicle can easily be stabilized through the steering control independent of the operation of the steering operation member. For example, a target yaw rate or a target lateral acceleration is determined in accordance with the operation torque or operation angle of the steering operation member, and the attitude control of the motor vehicle can be achieved by electrically controlling the operation of the steering mechanism on the basis of the target yaw rate or the target lateral acceleration.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.